The present invention pertains to microfabricated electromechanical (MEM) switches which are fabricated on a substrate, and particularly to those which are fabricated for integration into circuits utilizing typical CMOS processing steps.
MEM switches in various forms are well-known in the art. U.S. Pat. No. 5,121,089 to Larson, granted in 1992, describes an example of a MEM switch in which the armature rotates symmetrically about a post. That inventor also suggested cantilevered beam MEM switches, in xe2x80x9cMicroactuators for GaAsxe2x80x94based microwave integrated circuitsxe2x80x9d by L. E. Larson et al., Journal of the Optical Society of America B, 10, 404-407 (1993).
MEM switches are very useful for controlling very high frequency lines, such as antenna feed lines and switches operating above 1 GHz, due to their relatively low insertion loss and high isolation value at these frequencies. Therefore, they are particularly useful for controlling high frequency antennas, as is taught by U.S. Pat. No. 5,541,614 to Lam et al. (1996). Such MEM switches have been made typically using gold to provide metal for the contacts.
It is desirable to fabricate such antennas in an array, and thus the MEM switch controllers need to be in an array also. In order to reduce costs and simplify producing arrays of MEM switches using known techniques, it is desirable to make MEM switch construction compatible with CMOS processes. Gold is not available in typical CMOS fabrication processes. Aluminum has been used for MEM switch contacts with CMOS processing, but aluminum contacts suffer from a tendency to oxidize and to adsorb surface contaminants. Polysilicon has also been used, but is a material of very high resistivity and thus does not readily provide good contact connections.
Thus, there exists a need for MEM switches which are compatible with CMOS processes, and which have an improved contact system.
The present invention solves the problem of building MEM switches which are entirely compatible with standard integrated circuit processes, such as CMOS, and which yet have low resistance contacts with good high-frequency performance.
The present invention provides a method to fabricate high-performance MEM switches using standard metallization layer interconnect vias. In the preferred embodiment, which utilizes CMOS fabrication steps, aluminum metallization is used for RF transmission lines and mechanical structural elements, and tungsten plugs are used as contacts for the MEM switches. Tungsten contacts are not only less susceptible to oxidation and to adsorption of contaminants than is aluminum, but they also have higher annealing and melting temperatures, and are harder. Thus, tungsten contacts provide greater contact lifetime and higher current-carrying capacity than aluminum, and much lower resistance than polysilicon.
Tungsten is currently preferred in most multiple metallization layer CMOS processing, but the present invention is directed not only to the use of tungsten, but to the use of CMOS via material, whatever it might be, to form MEM switch contacts.